core/socket: introduce AcceptFileDescriptors=

core/socket: introduce AcceptFileDescriptors=

This controls the new SO_PASSRIGHTS socket option in kernel v6.16.
Note that I intentionally choose a different naming scheme than
Pass*=, since all other Pass*= options controls whether some extra
bits are attached to the message, while this one's about denying
file descriptor transfer and it feels more explicit this way.
And diverging from underlying socket option name is precedented
by Timestamping=. But happy to change it to just say PassRights=
if people disagree.